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Jan- 7, 1964 L. A. MEACHAM Re. 25,507

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a moo o E sa n D E Y loo 691 11o m 94a |094 |209 Issa |411 FREQUENCY cps2O l A o.| |.o |O |00 /NVENTOR cuRRL-Nr- Dc M/L/AMPERes L' ,4 MEAC/7AMam @QA A TTORNEY United States Patent O 25,507 TELEPHONE SUBSTATIONAPPARATUS Larned A. Meacham, Colts Neck, NJ., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a corporation of New YorkOriginal No. 3,064,084, dated Nov. 13, 1962, Ser. No. 859,936, Dec. 16,1959. Application for reissue Dec. 26, 1962, Ser. No. 247,799

7 Claims. (Cl. 179-81) Matter enclosed in heavy brackets [1 appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

This invention relates to telephone substation apparatus and moreparticularly to transistorized calling signal generators for telephonesets.

ln my patent application Serial No. 759, 474, filed September 8, 1958, ldisclose a multifrequency calling signal generator for telephonesubscriber sets employing a single transistor oscillator operative togenerate two selected frequencies for each digit upon the operation ofany of a series of pushbuttons. Ten such pushbuttons, each arranged toselect a different pair of frequencies, allow the coding of subscribercalling information.

As proposed in my earlier application, the calling signal generator is,in effect, connected across a resistor which, when a pushbutton isoperated, itself is connected in series with the two telephone lineconductors whereby power for the transistor oscillator is obtained fromthe voltage drop across that resistor. When no pushbutton is operated,the speech circuit is connected across the line conductors as instandard telephone practice and the telephone transmitter is likewisepowered by the supervisory direct current on the line, supplied from thetelephone central office.

It has been found that this arrangement is satisfactory for both speechand pushbutton signaling but certain limitations have been encountered.First, the series resistor absorbs fifty percent or more of thesignaling power so that it has been necessary to operate the callingsignal oscillator at a high level in order that this absorption alongwith the normal attenuation over the longest line still does not lowerthe level of the received calling signal below the point at which thepresence of interfering voltages such as dial tone can prevent itsdetection by the central office calling signal receiver. Such alimitation imposes rather severe operating conditions for the oscillatoron long loops and in particular produces a problem of excessivedissipation in the transistor on short loops.

Another diiculty occurs in connection with the aforemention change ofconnection effectively to replace the speech circuit with the resistorand transistor oscillator each time a pushbutton is operated. With thiscircuit change there is a change in the over-all set resistance from thenormal 200 ohms to approximately 600 ohms. Therefore, with eachpushbutton operation in addition to the two calling frequenciesgenerated and transmitted from the telephone set there is also astepwise reduction in the direct current on the telephone line. The dropin direct current can be sufficient to cause the central otliceconnection to be released. Also, the transient cornponents generated bythe direct current step and occurring simultaneously with the callingsignal constitute interference.

One further limitation in the multifrequency pushbutton callingarrangement is that the phenomenon known as frequency twist or unequalattenuation of different frequencies during transmission results in thetwo signaling components being of substantially different amplitudeafter transmission over a long loop. This difference Reissued Jan. 7,1964 ICS in amplitude handicaps the signal in competing with dial toneor other interference, as noted above, for proper detection by thecentral office receiver.

With these limitations in mind, it is a general object of this inventionto improve the operation of multifrequency telephone calling apparatus.

Another object of this invention is to increase the signaling efiicicncyof the pushbutton calling circuits in a telephone set.

Another object of this invention is to maintain a substantially uniformtelephone set resistance during both speech and pushbutton calling.

Still another object of this invention is provide frequency compensationfor multifrequency calling signals.

These objects are attained in accordance with this invention, oneembodiment of which is shown in the drawing. It comprises an integratedspeech and calling signal-generating circuit having a single pair ofline terminals to which the speech circuit including a transmitter, areceiver, an induction coil, and a line-balacing network are allconnected in conjugate relationship, as is standard in the telephoneart. Normally bridged across the induction coil through a transferswitch are a pair of seriesconnected tank circuits having selectedtuning taps under the control of an array of pushbuttons. The inductorof each tank circuit has two closely coupled additional windings,connected respectively in the emitter circuit and the base circuit of atransistor. This arrangement provides a path for coupling energy fromthe output to the input of the transistor to establish the conditionsfor oscillation generation. The transmitter of the telephone circuit isconnected through a back contact of the transfer switch whereuponoperation of any of the pushbuttons and consequent operation of thetransfer switch causes the transmitter circuit to be opened. Thereceiver is also connected through a back Contact of this switch, sothat the signal tones produced while a pushbutton is pressed do notsound excessively loud to the subscriber. At the same time theemitter-collector circuit of the transistor is connected across aportion of the induction coil and the line-balancing network of thespeech circuit for coupling energy out of the calling signal generatorand onto the telephone line.

One feature of this invention resides in the alternate connection of thetelephone transmitter and multifrequency calling signal generator intothe induction coil circuit with each operation of the calling signalpushbutton.

Another feature of this invention relates to the coupling of thetransistor oscillator to only a fractional portion of the induction coilto use the induction coil as an autotransformer during signaling.

One other feature of this invention is the coupling of the transistoroscillator to the anti-sidetone network to providefrequency-compensation for the calling signal generator.

Still another feature of this invention is the utilization of thenonlinear loop-equalizing elements of the speech transmission network assupply voltage limiters for the calling signal oscillator.

These and other features of this invention may be more clearlyunderstood from the following detailed description and by reference tothe drawing in which:

FIG. 1 is an electrical schematic representation of a telephonesubstation circuit in accordance With this invention;

FIG. 2 is a graphical showing of the electrical characteristics of anonlinear element employed in this invention; and

FIG. 3 is a graphical representation of the relative levels of callingsignals at the terminals of the subscriber installation.

Referring now to the drawing, FIG. 1 shows a telephone subscribercircuit connected through a pair of switchhook contacts 11 and 12 to apair of line conductors L1 and L2 to a telephone central office 13including a calling signal receiver of the type disclosed in my patentapplication Serial No. 743,434, lled June 20, 1958. Connected betweenthe line conductor L2 and ground is a conventional telephone ringer 14.

Branching from switchhook contact 11 is the calling signal generator 15hereinafter described, and bridged across the switchhook contacts 11 and12 is the induction coil L, including series-aiding windings 20, 21, and22, with a nonlinear resistance element 23, for example, a WesternElectric 3121 silicon carbide varistor in series between windings 2l)and 21. Another bridging path includes conductor 24, resistance 25, anda varistor 26 of a type similar to varistor 23, for example, a WesternElectric 312D varistor, and conductor 27. The nonlinear characteristicsof varistors 23 and 26 are employed for loop equalization; that is, tocontrol, as a function of loop length, both the transmitting andreceiving sensitivities of the telephone set, and at the same time tomaintain a required degree of sidetone suppression.

Connected to the junction between windings 21 and 22 of the inductioncoil L is the transmitter branch including a small series resistor 30,the telephone transmitter 31, a back contact 32 and the armature 33 oftransfer switch 34 as well as a conductor 35 terminating at the junctionbetween the varistor 23 and winding 20. The telephone transmitter 31 istherefore bridged across winding 21 and varistor 23 when the switch 34is in its unoperated condition` The varistor 23 is a nonlinear elementof the antisidetone network 36, which includes, additionally, shuntedacross this varistor, a small capacitor of approximately 0.04 microfaradand also a resistor 41 in series with a larger capacitor 42 in the orderof 2 microfarads.

The telephone receiver branch extends from terminal 43 of theanti-sidetone network 36 through winding 44 which is coupled inseries-aiding relationship to induction coil L, conductor 45, receiver46, shorting contact of switch 34 and the conductor 35 to junction 51.In the presence of speech currents at the transmitter, the voltageinduced in the winding 44 between the terminals 43 and 52, due tocoupling to the induction coil, is approximately equal to, and in phasewith, the voltage developed across the sidetone network terminals 43 and51. Consequently, the voltage between the junctions 51 and 52 acrosswhich the receiver is connected is approximately zero in the presence oflocal speech. This is a classic form of anti-sidetone speech circuitwith its success depending on the relationship between the impedance ofthe anti-sidestone network 36 and that presented to the set by the lineL1, L2.

In addition to the speech circuit described above, a branch from theswitchhook contact 11 over conductor extends to a pair ofseries-connected coils 61 and 62 with capacitors 66 and 67 arranged tobe connected across the respective coils 61 and 62 upon operation of oneof the calling pushbuttons, unshown, which closes one each ofcrosspoints 68 and 69. This branch extends through conductor 63, asecond back contact 64 of the switch 34, and conductor 35 to junction51. These windings 61 and 62 are therefore normally in shunt withwinding 20 of induction coil L. When the telephone set is connected tothe telephone line and the switch 34 is in its unoperated condition, aportion of the line current passes through the windings 61 and 62 whilethe majority of the line current passes through winding 20 of inductioncoil L. When the switch 34 is operated the path through windings 61 and62 is interrupted.

A second branch from conductor 60 constitutes the collector circuit 70of a transistor 71. The transistor 71 includes a base electrode 72 witha pair of series-connected windings 73 and 74 connected via conductor 75and a resistor 76 to the front contact 77 of switch 34. Individuallybridging the windings 73 and 74 are varistors 80 and 81, for example,two Western Electric 100A silicon varstors, the nonlinear resistancecharacteristics of which respectively control the magnitude of anyalternating current voltages developed across windings 73 and 74.Transistor 71 includes also an emitter electrode 82 connected through aresistor 83, series coils 84 and 85, the conductor 86 to the telephonetransmitter branch at junction 90. Coils 61, 73 and 84 are windings on acommon core and have close magnetic intercoupling. Similarly, coils 62,74 and 85 are closely intercoupled` Base bias for the transistor 71 isderived across the varistor 91 and resistor 92 which, together withresistor 76 form a voltage divider between points 51 and 90 when switch34 is operated.

The operation of the ringer and speech circuits of this substationcircuit are not modied from the prior art. In particular, the ringerwill be energized when a ringing signal is applied at the central officebetween ground and conductor L2.

In so far as the speech circuit is concerned, lifting of theconventional handset from the base of the telephone set allows theswitchhook contacts 11 and 12 to close and supervisory current to owthrough the line conductors L1 and L2 into the speech circuit throughthe winding 20 on conductor 35, the armature 33 and back contact 32 ofthe switch 34, the transmitter 31, resistor 3i), winding 22 and lineconductor L2. This direct current energizes the telephone transmitter31. Local speech at the transmitter is introduced into the inductioncoil branch between the terminals 51 and 93 and then by autotransformeraction employing windings 20, 21 and 22 into the line conductors L1 andL2. As indicated above, the speech voltage induced in winding 44 coupledto the induction coil is substantially equaled by the voltage drop inthe anti-sidetone network across terminals 43 and 51 and therefore thereis no voltage developed across the terminals 51 and 52. Incoming speechover the line L1 and L2 is not balanced out and reaches the receiver`uranch where it is audible to the listener at the receiver 46.

The calling mechanism employs a plurality of pushbuttons, at least tenin number, the operation of any one of which causes the generation oftwo tones, lasting as long as the button is held down. While a button isdepressed the speech circuit is disabled.

The operation of a pushbutton results in the actuation of threeswitches, that is, one of the crosspoints 68, one of the crosspoints 69,and lastly the common switch 34 which includes contast 50. In thedrawing switch 34 is shown in the normal (speech) condition.

Whether or not a pushbutton is pressed, operation of the telephone setdepends upon direct current supplied over the line conductors L1 and L2from the telephone central oce 13. Whenever the telephone handset islifted and the line contcats 11 and 12 are closed, direct current ofbetween 25 and 20() milliamperes, depending in part upon the length ofloop from the substation to the central office, flows through theconductors L1 and L2. On a normal or short loop the primary directcurrent path is from the line conductor L1 through line switch 11,conductor 17, induction coil windings 20, 21 and 22 and the intermediatevaristor 23, return conductor 18, switchhook contact 12 and lineconductor L2. An additional direct current path is through conductor 24,resistor 25, varistor 26 and conductor 27. These paths combine to givethe set a low resistance and thus to maintain a low voltage across theline conductors L1 and L2. Direct current is supplied selectively to thetransmitter 3l or to the transistor 71 from the voltage drop primarilyacross the varistor 23.

On short loops, the alternating-current resistance as wcll as thedirect-current resistance of both varistors 23 and 26 are reducedrelative to their corresponding resistances on long loops by theincreased current which they conduct. As a result, varistor 26 reducesthe transmitting and receiving sensitivities of the telephone set moreon short loops than on long, by acting as a variable shunt between theloop conductors L1 and L2. Simultaneously, the variation inalternating-current resistance of varistor 23 changes the effectiveimpedance of the anti-sidetone network 36 in such a way as to compensatefor the changes in effective line impedance produced by varistor 26 andthus preserve satisfactory sidetone suppression. This action, old in theart, is known as loop equalization. It tends to adjust station setsensitivities to correct for differences in transmission to the centraloffice.

During the off-hook but nonsignaling condition the collector current oftransistor 71 is effectively zero. The emitter and base of thetransistor 71 are at substantially the same voltage since no base biasis applied through Contact 77 of switch 34.

The principal direct current voltage drop in the entire telephone setoccurs between the terminals 51 and 59. On a maximum subscribed loopthis drop falls to about 4 volts. On an extremely short loop its rise isrestricted by the varistor 23 to about 10 volts. For collector currentsregulated at about milliamperes by the biasing diodes 91, excessivedissipation in the transistor is eliminated. The varistor 23 having aprimary function as a nonlinear element of the anti-sidetone network 36and the varistor 26 having a primary function as a nonlinear element toregulate speech transmission combine to serve the additional function ofregulating the collector voltage and hence the power which must bedissipated during pushbutton signaling by transistor 71.

Whenever a pushbutton is operated the two taps corresponding to the pairof frequencies indicative of the digit are closed and immediatelythereafter switch 34 is actuated. The operation of transfer switch 34opens the transmitter circuit by the opening of contact 32, insertsattenuation in the receiver circuit by the opening of contact 50 whichnormally shorts resistor 57, enables the transistor oscillator by theestablishing of a base bias on a closing of contact 77, andshock-excites both coils 61 and 62 into oscillation by interrupting thedirect current through them upon opening of contact 64. The transistor71, acting as a Class A amplifier, sustains the shockexcitedoscillations at amplitudes regulated by varistors 8l) and 81, and theseoscillations are coupled into the induction coil and thence to the lineconductors L1 and L2.

Output is taken from the transistor between the emitter circuitconductor 86 and resistor 30 at terminal 93 and the collector circuitthrough conductor 60. The voltage amplitude of the calling signals isincreased by the autotransformer action of the windings 20, 21 and 22 ofthe induction coil. With some loss introduced into the calling signal bythe presence of resistor 30 and the varistor 23, the induction coilstill affords approximately 1.4 to 1 increase in amplitude, therebyallowing the transistor 71 to be operated at lower alternating currentvoltage and hence lower direct-current collector voltage for the samecalling signal levels as heretofore were available.

The presence of the anti-sidetone network 36 including. in particular,the varistor 23, having the characteristics shown in FIG. 2, in thecoupling between the calling signal generator and the line has a furtheradvantageous effect. As indicated above, the varistor 23 introduces someattenuation into the transistor calling signals. This varistor 23 isshunted by a capacitor 40 which offers a degree offrequency-compensation in the calling signal. Over the range of callingsignals of 679 through 1477 cycles per second there is approximately 2decibels difference in level of the terminal voltage owing to thepresence of the attenuating varistor 23 shunted by the capacitor 40. Theintroduction of this pre-emphasis of higher signaling frequencies isadvantageous particularly on long loops where they are attenuated morethan the lower frequencies. An additional pre-emphasis of about 1decibel is applied by choice of the numbers of turns of the coils 73, 74across which varistors 80 and 81 are bridged. The over-all effect ofthese compensations may be seen in FIG. 3 which is a plot of therelative amplitudes versus signaling frequencies.

It may be seen from the above description that the calling transmitterof this invention is supplied with power from the voltage developedacross a portion of the antisidetone network of the speech circuit andsimilarly is afforded also a frequency-compensation by the anti-sidetonenetwork.

The induction coil formerly associated solely with the speech circuit intelephone substation installations additionally constitutes anautotransformer for increasing the amplitude of calling signals. Theoverall telephone set resistance remains now substantially constantduring signaling and nonsignaling conditions, since (l) the supplyvoltage required to generate the signals is reduced, thus permittingthis resistance to be low during signaling; (2) the induction coilbalancing network and anti-sidetone network all are present during bothconditions, (3) the only change affecting the set resistance is theselection of alternating signal sources, to wit, the carbon transmitteror the oscillator, and (4) the resistance effects of this selection areminimized by the nonlinear elements of the anti-sidetone network.

In all cases it is understood that the above-described arrangements aremerely illustrative of the principles of the invention. Numerous andvaried other embodiments may be devised in accordance with theseprinciples by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is:

1. A telephone substation circuit comprising a line impedance balancingnetwork, said balancing network including a nonlinear impedance element,[a direct-current path including said nonlinear impedance element,] adirect-current path including said nonlinear impedance elcment, amultifrequency generator comprising a resonant circuit having discretelycontrollable resonant frequencies and amplifying means [having input andoutput terminals, said output terminals being coupled to saiddirectcurrent path], means for selecting a discrete frequency of saidresonant circuit, said resonant circuit being coupled to said amplifyingmeans in such manner as to render said simplifying means capable ofproducing oscillations at said selected frequency, switch means normallyconnecting said resonant circuit across [said nonlinear impedanceelement] a portion of said direct-Current pat/1, said switch means beingactuated by the operation of said discrete frequency selecting means tointerrupt said normal connection and connect [the input terminals of]said amplifying means t0 derive operating voltage from the voltage dropacross said nonlinear impedance element, and means coupling the outputof said multifrequency generator to said direct-current path.

2. A telephone substation circuit including a line impedance balancingnetwork, an induction coil comprising a plurality of windings disposedin such manner as to provide autotransformer action, said balancingnetwork including a nonlinear impedance element and a capacitor in shuntwith said nonlinear impedance element, a directcurrent path including[the windings] a winding of said induction coil and said [non-linear]nonlinear impedance element, a multifrequency generator comprising aresonant circuit having discretely controllable resonant frequencies andamplifying means [having input and output terminals, said outputterminals being connected across a winding of said induction coil],means for selecting a discrete frequency of said resonant circuit, saidresonant circuit being coupled to said amplifying means in such manneras to render said amplifying means capable of producing oscillations atsaid selected frequency, switch means normally connecting said resonantcircuit across [said nonlinear impedance element] a portion of .middirect-current path, said switch means being actuated by the operationof said discrete frequency selecting means to interrupt said normalconnection and [connect the input terminals of] apply to said amplifyingmeans current derived from tlze voltage drop across said nonlinearimpedance element, and means connecting the output of saidrnultifrequency generator across a winding of said induction coil- 3. Atelephone substation circuit comprising a transmitter, an anti-sidetonenetwork, an inductive element, said anti-sidetone network including a[non-linear] nonlinear impedance element, a direct-current pathincluding said nonlinear impedance element and said inductive elemcnt, amultifrequency generator comprising a [rsonant] resonant circuit havingdiscretely controllable resonant frequencies and amplifying means[having input and output terminals, said output terminals beingconnected across said inductive elementl, means for selecting a discretefrequency of said resonant circuit, said resonant circuit being coupledto said amplifying means in such `manner as to render said amplifyingmeans capable of producing oscillations at said selected frequency,switch means normally connecting said resonant circuit across a portionof said direct-current patlz and said transmitter across said nonlinearimpedance element, said switch means being actuated by the operation ofsaid discrete frequency selecting means to interrupt said normalconnections and [connect the input terminals of] enable said amplifyingmeans wit/1 Operating voltage derived from tlte voltage drop across saidnonlinear impedance element, and means connecting tlle output of saidmultifrcquency generator across said inductive element.

4. A telephone substation circuit including a transmitter, ananti-sidetone network, an induction coil comprising a plurality ofwindings disposed in such manner as to provide autotransformer action,said anti-sidetone network including a nonlinear impedance element and acapacitor in shunt with said nonlinear impedance element, adirect-current path including [the windings] a winding of said inductioncoil and said nonlinear impedance element, a multifrequency generatorcomprising a resonant circuit having discretely controllable resonantfrequencies and amplifying means [having input and output terminals.said output terminals being connected across a winding of said inductioncoil], means for selecting a discrete frequency of said resonantcircuit, said resonant circuit being coupled to said amplifying means insuch manner as to render said amplifying means capable of producingoscillations at said selected frequency, switch means normallyconnecting said resonant circuit across a portion of said direct-currentpath and said transmitter across said nonlinear impedance element, saidswitch means being actuated by the operation of said discrete frequencyselecting means to interrupt said normal connections and [connect theinput terminals of] apply to said amplifying means current derived fromthe voltage drop across said nonlinear impedance element, and meansconnecting the output of said multifrequency generator across a windingof said induction coil.

5. A telephone substation circuit including a transmitter, ananti-sidetone network, an induction coil comprising a plurality ofwindings disposed in such manner as to provide autotransformer action,said anti-sidetone network including a nonlinear impedance element and acapacitor in shunt with said nonlinear impedance element, adirect-current path including [the windings] a first and a secondwinding of said induction coil and said nonlinear impedance elementserially connected, a multifrequency generator comprising a resonantcircuit having discretely controllable resonant frequencies andamplify'- ing means [having input and output terminals, Said outputterminals being connected across a winding of said induction coil andsaid nonlinear impedance element, said winding and nonlinear impedanceelement being serially connectedj, means for selecting a discretefrequency of said resonant circuit, said resonant circuit being coupledto said amplifying means in such manner as to render said amplifyingmeans capable of producing oscillations at said selected frequency,switch means normally connecting said resonant circuit across a windingof said induction coil in said direct-current pat/i and said transmitteracross said nonlinear impedance element and [said] o winding of saidinduction coil in the direct-current put/r, said switch means beingactuated by the operation of seid discrete frequency selecting means tointerrupt said normal connections and connect [the input terminals of]said amplifying means to derive operating voltage fro/n tlie voltagedrop across said nonlinear impedance element and [said] o winding ofsaid induction coil in said directcurrent pat/t, and means connectingthe output of said tnultifrequency generator across said nonlinearimpedance element and tz winding of said induction coil iu suitldirect-current pat/t.

6. A telephone substation circuit comprising a line impedance balancingnetwork, sold balancing network including a nonlinear impedance element,a direct-eurrent patlz including said nonlinear impedance element, amultifrequency generator comprising a resonant circuit having discretelycontrollable resonant frequencies and amplifying means, means forselecting a discrete f rcquency of said resonant circuit, said resonantcircuit being coupled across a portion of said direct-current pat/1 forstorage of energy in said resonant circuit and coupled to saidamplifying means in suc/t manner as to render salti amplifying meanscapable of producing oscillations at said selected frequency, switchmeans actuated by tlze operation of said discrete frequency selectingmeans for releasing said stored energy to shock-excite said resonantcircuit and enabling said amplifying means with operating voltagederived from the voltage drop across said nonlinear impedance element,and means coupling the output of said multifrequency generator to saiddirect-current path.

7. A. telephone substation circuit comprising a line impedance balancingnetwork, said balancing uetufor/tY including a nonlinear impedanceelement, a direct-current pat/t including said nonlinear impedanceelement, a multifrequency generator comprising a resonant circuitltoring discrete/y controllable resonant frequencies and amplifyingmeans, means for selecting a discrete frequency of said resonantcircuit, said resonant circuit being coupled to said amplifying means insuch manner as to reuder said amplifying means capable of producingoscillations at said selected frequency, switch means actuated by theoperation of said discrete frequency selecting means' for applying tosaid amplifying means current dcrved from tlze voltage drop across saidnonlinear impedance element, and means coupling tlte output of saidmultifrequcncy generator to said direct-current path.

C'lemency July 30, 1957 Crofutt Aug` 5, 1958

